Border Wardens: What Are The Borderlands?

When I first picked up the Keep on the Borderlands, I skipped right past the Keep, and the outdoor map, to the Caves of Chaos.

Now that I've returned, different things catch my eye as hooks:

• the various factions of the Keep, and the intrigue unfolding there
• the two (?) references to the Caves of the Unknown
• the mad hermit, the lizardmen, the spiders, and the raiders in the wilderness

And I begin to read things into the setting, especially tied to the setting of Mystara.

Why is the Castellan here? He's clearly meant to be five levels higher than any of the PCs, as are many of the high positions in the Keep -- what is he meant to guard?

1. A more prosaic reason: this is where an invasion from the western mountain border might osttensibly take place, hence the fortifications and quite possibly some kind of early warning system back to the capital. We should expect more secrets: hidden weapons and secret allies from the wilderness, as well as a truly marvelous method of notifying the nearest military forces of Karameikos.
2. A more fantastic reason: the keep guards against encroaching Chaos. The Caves of Chaos were so named by a powerful (and now deceased) duo of magic-user and cleric. Naming it (in a magical/theurgical sense) thereby curtailed what it used to be -- a font of uncontrollable forces of Chaos from the mountains, or the earth, or a rift in reality. The Castellan (and those who hold official positions in the Keep) were also named, and gained special abilities when in opposition to those forces. Many fell in the Forgotten War, and they always keep an eye out for trustworthy adventurers to induct into their ranks. And they keep an eye on the ever-shifting tactics of Chaos.